As particle preparation techniques, general preparation methods include a double emulsion method (W/O/W), a phase separation method, a spray drying method, a super critical fluid (SCF) method, and the like. In recent years, a method of preparing monodispersed particles has been developed by which uniform particles can be prepared.
A representative example of the double emulsion method includes a method of preparing fine particles, which includes forming a primary emulsion by dissolving a biodegradable carrier in a solvent and dissolving a water-soluble drug in an aqueous phase, and then forming particles by injecting the primary emulsion into a secondary solution in which poly(vinyl alcohol) (PVA) is dissolved. Takeda corporation registered patent (U.S. Pat. No. 4,652,441) regarding the double emulsion method, and many other patents were filed regarding the double emulsion method after mass production and commercialization. Particularly, methods have been developed of preparing fine particles whose encapsulation ratio is improved according to a drug and which exhibit initial release control by optimizing the composition, molecular weight, change in terminal group, and the like of a polymer configuring the fine particles, and have been known as the most suitable methods up to for the purposes of encapsulating the water-soluble drug. However, as disadvantages, the particles exhibit porosity due to temperature, concentration, and stirring speed during the preparation process, which leads to an increase in the initial drug release ratio and a decrease in the encapsulation ratio of the drug. Further, as a solvent dissolving the polymer, methylene chloride having high toxicity is used, and its water solubility is as low as 20 g/L (20° C.). Since an organic solvent having low water solubility needs to be used to encapsulate the water-soluble drug effectively, solvents that may be selected for dissolving the polymer are limited.
With respect to the phase separation method, a patent (U.S. Pat. No. 4,460,563) was first filed in 1984, and its related products includes Somatuline LA. The phase separation method is a preparation method including forming the primary emulsion into a coacervate with silicon oil and the like and then forming particles by curing the polymer. In recent years, a technical trend of the phase separation method is to control the particle size, encapsulation ratio, and the like of the fine particles according to changes in pH, temperature, and compositions of the coacervate. The phase separation method has an advantage in that the size of the particles may be controlled only by controlling a chemical composition without any need for specialized facilities. However, it has a disadvantage in that a large amount of solvent needs to be used, and it has many parameters when the coacervate is formed.
The spray drying method is a method of forming fine particles by spraying a primary emulsion in hot air. A method was developed by Alkermes Corporation in which the primary emulsion is sprayed in hot air to compensate for the disadvantage that there is a limitation in use of encapsulated drugs, quenched using LN2, and extracted with ethanol. After development of the freeze-spray drying method, similar techniques have been developed so far. Various shapes of spray nozzles have been developed, and fine and uniform particles may also be formed using an atomizer, an electrode, a sonicator, and the like. The spray drying method has an advantage in that the particles may be easily mass-produced, but has a disadvantage that it is difficult to form spherical particles and the primary emulsion needs to be sprayed in hot air except in the freeze-spray drying method.
The super critical fluid (SCF) extraction method is a technique of spraying the primary emulsion with a super critical fluid by using a representative supercritical fluid CO2 as an anti-solvent. The SCF extraction method has an advantage in that it is eco-friendly and does not require an additional process of removing a residual solvent and the like, but has a disadvantage in that it is difficult to control particle size distribution during mass production.
When the particles are prepared using a conventional double emulsion method, the step of forming particles by injecting the primary emulsion into the aqueous medium and diffusing the solvent used in the primary emulsion into the aqueous medium is very important in preparing the particles. In the step, particle size, porosity, surface roughness, encapsulation ratio, and the like of the fine particles are determined according to the speed of the solvent which is diffused into the aqueous medium. The solvent diffusion speed is determined according to the kind of solvent, water-solubility of the solvent, temperature of the aqueous medium, and the kind, concentration, stirring speed, and the like of a surfactant configuring the aqueous medium.
In the above step, in order to increase the speed at which the solvent is diffused into the aqueous medium, a method of selecting a solvent having high water-solubility, increasing a temperature of the aqueous medium, reducing a concentration of the surfactant configuring the aqueous medium, or the like may be used. When the solvent diffusion speed is increased, the curing time of the polymer constituting the carrier is shortened, and thus the porosity of the particles is increased, the particle size is relatively large, and the encapsulation ratio is decreased. As a result, a solvent having high water-solubility such as ethyl acetate is diffused into the aqueous medium at a rapid speed, resulting in difficulty in forming a spherical shape, a low encapsulation ratio, and easily-formed porous particles. Accordingly, when the solvent having high water-solubility is used, the concentration of the aqueous medium is increased or the temperature is decreased to prevent the solvent from being diffused to the aqueous medium at a rapid speed. In U.S. Pat. No. 6,565,777, a multistep method of preparing fine particles using ethyl acetate is disclosed. This is a method of forming particles by initially reducing the volume of the aqueous medium to prevent ethyl acetate from being diffused into the aqueous medium at a rapid speed before forming the fine particles, and then evaporating the residual solvent by diluting the particles with a large amount of aqueous medium. However, it is disadvantageous in that a yield is reduced to 10 to 20%. Accordingly, in order to form the particles by various methods without limiting the solvent, the demand for a technique capable of controlling solvent diffusion time is very high.
In order to cure the polymer and diffuse the solvent into the aqueous medium, generally, a stirring process is performed. The stirring process includes methods such as stirring at room temperature, heated stirring, pressure stirring, and the like. In the case of stirring at room temperature, stirring time during mass production is increased, and a water-soluble drug is released after stirring for a long time, which leads to a decrease in the encapsulation ratio. During heated stirring, the curing speed of the polymer is increased and the stirring time is reduced, but the polymer exhibits porosity and its content is decreased. During pressure stirring, the solvent is diffused into the aqueous medium and simultaneously evaporated by pressure, and thus the solvent may be evaporated in a rapid time, and fine particles appropriate under the guidelines on residual solvents in pharmaceuticals may be prepared. However, during pressure stirring, when the reaction volume is large, it is difficult to reach a desired vacuum and the degree of vacuum is decreased, and the polymer needs to be carefully treated to prevent formation of bubbles in the solvent during stirring under the control of the vacuum. Particularly, when the surfactant is included, the bubbling phenomenon is more serious, and thus conditions need to be maintained to control a vacuum pressure. In order to solve these problems, in U.S. Pat. No. 7,029,700, a method of cooling the entire aqueous medium and removing the solvent under depressurization was used. Further, in U.S. Pat. No. 6,020,004, a preparation method of obtaining microspheres by cooling and freeze-drying the primary emulsion was disclosed.
Under the background, the present inventors have confirmed that microspheres having a spherical shape may be obtained by using a polymer having a sol-gel transition property as a surfactant included in an aqueous medium into which a primary emulsion is injected in order to gelate a secondary emulsion formed after injecting the primary emulsion, thereby preventing a solvent in a polymer solution for a carrier from being rapidly diffused into the aqueous medium during formation of the microspheres to reduce porosity of the microspheres and reduce surface roughness of the microspheres, and a high encapsulation ratio of the microspheres may also be easily achieved without restriction to solvent selection, or other stirring or solvent evaporation. Therefore, the present invention has been completed based on these facts. Further, in the method of preparing microspheres according to the present invention, since a desired amount of microspheres may be obtained at a low volume ratio of the secondary emulsion to the primary emulsion, innovative process efficiency may be improved through reduction of the reaction volume.